Oncology is the branch of medicine directed to the study of the development, diagnosis, treatment, and prevention of tumors. A tumor is an abnormal growth of tissue serving no physiological function. A tumor may be malignant (cancerous) or benign. A malignant tumor may exhibit uncontrolled, progressive multiplication of cells and spread cancerous cells to other parts of the body (metastasizes) through blood vessels or the lymphatic system. A benign tumor does not metastasize, but can still be life-threatening if it impinges on critical body structures such as nerves, blood vessels, and organs.
Radiosurgery and radiotherapy are radiation treatment systems that use external radiation beams to treat tumors and other lesions by delivering a prescribed dose of radiation (e.g., x-rays, protons, or gamma rays) to a target volume (region of interest, or ROI) while minimizing radiation exposure to the surrounding tissue. The object of both radiosurgery and radiotherapy is the destruction of abnormal tissue while sparing healthy tissue and critical structures. Radiotherapy is characterized by a low radiation dose per treatment and many treatments (e.g., 30 to 45 days of treatment). Radiosurgery is characterized by a relatively high radiation dose to a tumor in one, or at most a few, treatments. In both radiotherapy and radiosurgery, the radiation dose is delivered to the tumor site from multiple angles. As the angle of each radiation beam is different, each beam passes through a different area of healthy tissue on its way to the tumor. As a result, the cumulative radiation dose at the tumor is high, while the average radiation dose to the surrounding healthy tissue is low. Unless specified otherwise, radiosurgery and radiotherapy are used interchangeably in the present application.
Radiation treatment systems may be used together with an imaging system for image-guided radiation therapy (IGRT). The imaging system acquires in-treatment images, e.g., x-ray, ultrasound, CT, or PET, that may be used to for patient set up and in some instances (e.g., Accuray Incorporated's CyberKnife® Radiosurgery System) guide the radiation delivery procedure and track in-treatment target motion. Target motion tracking may be accomplished by correcting for differences in target position by acquiring and registering intratreatment images with reference images, known as digitally reconstructed radiographs (DRRs), rendered from a pre-treatment computed tomography (CT) scan, which may otherwise be known as the treatment planning image.
Previously-known IGRT systems use imaging systems that generate single energy x-ray images during treatment. Such systems suffer, however, from a variety of drawbacks. For example, x-ray attenuation characteristics are dependent on x-ray energy and thus a single energy x-ray image may have limited differentiation ability for certain materials. An x-ray image generated using a low x-ray energy (e.g., ˜50-100 kV) will display significant attenuation in soft tissue and radio-opaque objects such as skeletal structures, fiducials, and contrast agents. Conversely, an x-ray image generated using a high x-ray energy (e.g., ˜100 kV-6 MV, preferably 100-150 kV) will display less attenuation of soft tissue than a low x-ray energy image, but still significant attenuation in radio-opaque objects such as skeletal structures, contrast agents, and fiducials.
In view of the above-noted drawbacks of previously-known systems, it would be desirable to provide apparatus and methods for generating x-ray images with increased discrimination between soft tissue and radio-opaque objects.
It further would be desirable to provide systems and methods for processing and using such x-ray images in image-guided radiation therapy.